We use techniques of classical immunogenetics and of molecular biology to study the genetics of rabbit immunoglobulins (Igs ), T cell receptors (Tcr), and related genes such as the recombinase activating genes RAG-1 and RAG-2 which are necessary for gene rearrangements to occur during lymphocyte development. We investigate the development of germinal centers and the regulated expression and sequence diversification of Ig genes during lymphoid cell development. Four VH-CH recombination sites map 3' of the VH genes and 5' of the JH genes. There appear to be two or more "hot-spots of recombination" within or near stretches of repetitive DNA in the DH-containing region of the rabbit. A fifth recombinant (R7K) has a site that maps 3' of the exons encoding the membrane terminus of the IgM heavy chain (Cmu) and 5' of Cgamma. The exact site of this recombination and its possible localization in a region homologous to the region in other species containing exons for the IgD heavy chain is currently under investigation. In contrast to most mammals studied, membrane-IgD has not been definitively identified on rabbit B cells. We have identified two surface immunoglobulin complexes on rabbit peripheral blood B cells that possess heavy chains of similar apparent molecular size. These may represent complexes with rabbit IgM and IgD. Four proteins are found noncovalently associated with the mIg in each of the receptors. These include 42, 37 and 36 kD proteins that may be the rabbit homologues of murine Ig-beta (B29) and Ig-gamma and Ig-alpha (mb-1). These proteins are found associated as 75 kd and 115 kD heteromeric complexes. Two larger glycoproteins (100 and 150 kD) may be specifically associated with the non-mu receptor. The Ig-associated proteins vary in B-lymphocytes isolated from spleen, blood, bone marrow and gut associated lymphoid tissue (GALT). Lectins are allowing us to dissect rabbit GALT into subcompartments based on local accumulations of cells expressing particular oligosaccharides. Normal and VH-expression mutant ali rabbits show similar staining patterns;the defect in ali rabbits which leads to the expression of a different set of VH genes does not appear to affect the general architecture of GALT.